Conductivity detection is used widely in ion chromatography (Gjerde and Fritz, Ion Chromatography, 2nd ed., Verlag: Heidelberg, 1987; and Smith, Ion Chromatography Applications, CRC Press: Boca Raton, Fla., 1988), but rarely in capillary zone electrophoreses (CZE) (Kuhr, Anal. Chem., 1990, 62 403R-414R). This difference apparently stems from the difficulty of fabricating a conductivity detector with low dead volume in the structure of a fused-silica capillary with an inside diameter of 100 .mu.m or less. Three possible designs for a CZE conductivity detector are on-column, off-column, and end-column structures. Foret et al. used a microscale molding method to construct an on-column conductivity detector which was also used on-line with a UV absorbance detector (Foret et al., Electrophoresis, 1986, 7, 430-432). Another scheme for constructing an on-column conductivity detector employs a focused CO.sub.2 laser beam to punch holes directly through the walls of the fused-silica capillary (Huang et al., Anal. Chem., 1987, 59, 2747-2749). Two platinum sensing electrodes are sealed on opposite sides of the column. Because CZE typically has a voltage drop of 300 V/cm along the length of the capillary, the sensing electrodes need to be aligned carefully and an isolation transformer must be used in measuring the conductance (Huang et al., Anal. Chem., 1987, 59, 2747-2749; and Everaerts et al., Isotachophoresis, Journal of Chromatography Library 6, Elsevier: Amsterdam, 1976).
Off-column conductivity detectors may be constructed by grounding the capillary prior to the sensing electrode, using either a porous glass structure (Wallingford and Ewing, Anal. Chem., 1987, 59, 1762-1766) or an on-column frit structure (Huang and Zare, Anal. Chem., 1990, 62, 443-446). Recently, Huang et al. reported the use of an end-column structure for conductimetric and amperometric detection in CZE in which the sensing electrode is placed at the outlet of the fused-silica capillary (Huang et al., Anal. Chem., 1991, 63, 189-192). This end-column structure is also described in U.S. patent application Ser. No. 07/580,259, filed Sep. 10, 1990, Huang et al. Such end-column detectors demonstrate sensitivities that approach those of previous on-column conductivity detectors with only a small sacrifice in resolution, and extra band broadening of approximately 25%. This end-column structure, however, requires carefully matched microplumbing in which the analytical capillary is placed inside a second capillary that has an inside diameter slightly larger than the outside diameter of the analytical capillary. Moreover, for conductimetric detection, the end-column structure employs epoxy to help maintain structural integrity of the electrode. Unfortunately, the epoxy becomes exposed to the electrolyte which can adversely affect measurements.